PanOph

LASIK

Cataract & Refractive Surgery

Key Points

  • •The minimum residual stromal bed (RSB) must be ≥ 250 μm — the most critical safety threshold to prevent post-LASIK ectasia.
  • •Dry eye is the most common complication of LASIK, caused by corneal nerve transection during flap creation; usually resolves by 6–12 months.
  • •Ectasia is the most feared long-term complication — screen rigorously with Pentacam BAD-D and corneal biomechanics to detect subclinical keratoconus.
  • •Patient satisfaction exceeds 95% (PROWL studies, FDA); efficacy: ~90% achieve 20/20 UCVA; >99% achieve 20/40 or better.
  • •Post-LASIK patients have falsely low IOP on Goldmann applanation due to thinner corneas — use corrected IOP or dynamic contour tonometry.
  • •The Munnerlyn formula (depth = S² × D / 3) calculates ablation depth — approximately 12–16 μm per dioptre of myopic correction.
1. Definition

Laser-Assisted in Situ Keratomileusis (LASIK) is a corneal refractive surgical procedure that uses a microkeratome or femtosecond laser to create a hinged corneal flap, followed by excimer laser (193 nm ArF) ablation of the exposed stromal bed to reshape the cornea and correct refractive errors including myopia, hyperopia, and astigmatism. It is the most commonly performed elective surgical procedure worldwide.

2. Epidemiology

Most commonly performed elective surgical procedure worldwide, with over 40 million procedures since its introduction in the early 1990s. At peak, approximately 700,000–800,000 LASIK procedures performed annually in the United States (volumes have declined in recent years). Peak age group: 25–40 years. Patient satisfaction rates exceed 95% in large meta-analyses (Sandoval et al., 2016; PROWL studies, FDA). Most commonly performed for myopia (-1.00 to -10.00 D), with or without astigmatism.

3. Aetiology and causes

Indications (refractive errors treated):

  • Myopia: up to -10.00 to -12.00 D (FDA-approved range varies by platform)
  • Hyperopia: up to +4.00 to +6.00 D
  • Astigmatism: up to 5.00–6.00 D
  • Mixed astigmatism

Mechanism of refractive correction:

  • Myopia: Central corneal flattening by removing more tissue centrally than peripherally (reduces corneal power)
  • Hyperopia: Peripheral corneal steepening by removing more tissue in the mid-periphery (increases central corneal power)
  • Astigmatism: Selective tissue removal along the steeper meridian to create a more spherical cornea
4. Pathogenesis

Surgical mechanism (biomechanical basis):

  1. Flap creation: A microkeratome (mechanical blade) or femtosecond laser (preferred) creates a corneal flap of ~90–120 μm thickness with a superior or nasal hinge.
  2. Femtosecond laser flap: Creates a precise lamellar cut by focusing ultrashort (10⁻¹⁵ second) infrared laser pulses (typically 1053 nm) into the stroma, producing photodisruption. Creates millions of microscopic gas bubbles that coalesce into a cleavage plane.
  3. Excimer laser ablation: The 193 nm ArF excimer laser breaks C-C and C-N molecular bonds in the corneal stroma via photoablation. Each pulse removes approximately 0.25 μm of tissue. A 6.0–6.5 mm optical zone is typical, with a larger transition/blend zone.
  4. Tissue removal rate: Approximately 12–16 μm of central tissue per dioptre of myopic correction (Munnerlyn formula: ablation depth = S² × D / 3, where S = optical zone diameter in mm, D = dioptres of correction).
  5. Wound healing: The flap adheres by natural suction and epithelial healing at the flap edge. Stromal wound healing is minimal (unlike PRK), which accounts for both the rapid visual recovery and the lifelong risk of flap displacement.
5. Classification

By flap creation method:

  1. Microkeratome LASIK: Mechanical blade (Moria, Hansatome). Meniscus-shaped flap (thinner centrally). Largely superseded.
  2. Femto-LASIK (all-laser LASIK): Femtosecond laser flap (IntraLase/iFS, VisuMax, LenSx, FEMTO LDV). Planar flap of uniform thickness. Current standard of care.

By ablation profile:

  1. Conventional (standard) LASIK: Sphero-cylindrical correction only.
  2. Wavefront-guided (custom) LASIK: Corrects higher-order aberrations (HOAs) based on wavefront analysis (aberrometry). Reduces night vision symptoms.
  3. Wavefront-optimised LASIK: Pre-compensates for spherical aberration induced by ablation without full custom measurement.
  4. Topography-guided LASIK (Contoura Vision, T-CAT): Ablation based on corneal topography data. Useful for irregular corneas and enhancement procedures.

By technique:

  • Standard LASIK
  • Thin-flap LASIK (sub-Bowman keratomileusis, SBK): 90–100 μm flap
  • Presbyopia-correcting LASIK: monovision (dominant eye distance, non-dominant near) or multifocal ablation profiles
6. Risk factors and associations

Contraindications (absolute):

  • Keratoconus or forme fruste keratoconus (ectasia risk)
  • Predicted residual stromal bed (RSB) < 250 μm (ectasia risk threshold — the primary absolute contraindication)
  • Corneal thickness < 480 μm (generally insufficient to achieve adequate RSB after ablation)
  • Unstable refraction (change > 0.50 D in past 12 months)
  • Age < 18 years (refraction not stable)
  • Active corneal disease (herpes simplex keratitis, corneal dystrophies)
  • Uncontrolled autoimmune disease (RA, SLE, Sjogren — abnormal wound healing)
  • Pregnancy or lactation (hormonal refractive changes)

Relative contraindications:

  • Thin corneas (480–500 μm): consider PRK or SMILE instead
  • Large pupil size > 7 mm in scotopic conditions (risk of night vision symptoms)
  • Dry eye disease (pre-existing moderate-severe)
  • Previous herpes simplex keratitis (recurrence risk)
  • High myopia > -10 D (insufficient tissue, consider ICL)
  • Significant corneal scarring
  • Uncontrolled glaucoma or glaucoma suspect
  • Diabetes mellitus (delayed epithelial healing)

Risk factors for poor outcomes:

  • Thin flap or buttonhole during creation
  • Decentred ablation
  • Inadequate residual stromal bed
  • Abnormal corneal topography (asymmetric bowtie, skewed radial axes)
7. Clinical features

Pre-operative evaluation findings (what to assess):

  • Uncorrected and best-corrected visual acuity
  • Manifest and cycloplegic refraction (must be stable × 12 months)
  • Corneal topography: symmetric bowtie pattern normal; asymmetric bowtie, inferior steepening, or skewed axes suggest ectasia risk
  • Corneal pachymetry: central corneal thickness (CCT) must allow adequate RSB (≥ 250 μm)
  • Pupillometry: scotopic pupil diameter (relevant for optical zone planning)
  • Tear film assessment: Schirmer test, TBUT, fluorescein staining
  • Slit-lamp examination: rule out corneal dystrophy, EBMD
  • Dilated fundus exam: rule out peripheral retinal pathology (lattice degeneration, tears)
  • Wavefront aberrometry: map higher-order aberrations

Post-operative normal course:

  • Day 1: mild foreign body sensation, tearing, mild haze. UCVA typically 20/40 or better
  • Week 1: UCVA approaches 20/20, mild dry eye common
  • 1–3 months: stabilisation of refraction. Mild fluctuation is normal.
  • 3–6 months: final refractive outcome. Dry eye usually resolves.

Abnormal post-operative findings (complications):

  • Flap striae (micro/macrostriae)
  • Diffuse lamellar keratitis (DLK) — "Sands of Sahara"
  • Epithelial ingrowth
  • Ectasia
  • Persistent dry eye
8. Investigations

Pre-operative workup:

  • Corneal topography (Placido disc-based: Orbscan, Atlas; Scheimpflug: Pentacam, Galilei): Detect subclinical keratoconus. Evaluate anterior and posterior corneal surfaces. Belin-Ambrósio Enhanced Ectasia Display (BAD) on Pentacam.
  • Corneal pachymetry: Ultrasonic or Scheimpflug-based. CCT and thinnest point. Calculate predicted RSB (must be ≥ 250 μm).
  • Wavefront aberrometry (Hartmann-Shack: WaveScan, iDesign, Zywave): Map HOAs for custom ablation.
  • Scheimpflug imaging (Pentacam): Corneal thickness map, anterior/posterior elevation maps, ectasia screening indices (BAD-D, Belin ABCD keratoconus staging).
  • Corneal biomechanics (Ocular Response Analyzer, Corvis ST): Corneal hysteresis (CH) and corneal resistance factor (CRF). Low CH associated with higher ectasia risk.
  • Pupillometry: NeurOptics or similar device for scotopic and mesopic pupil size.
  • Tear film assessment: Schirmer I (< 5 mm suggests severe dry eye), TBUT, osmolarity.
  • Manifest and cycloplegic refraction.
  • Contact lens holiday: Soft CLs removed 1–2 weeks before evaluation; RGP CLs: minimum 3–4 weeks; longer holidays (4–6 weeks or 1 week per decade of wear) recommended for long-term RGP wearers.
9. Differential diagnosis

Alternative refractive procedures (procedure selection, not disease differential):

  1. PRK (Photorefractive Keratectomy): Surface ablation without flap. Preferred for thin corneas, EBMD, contact sports, military/police. Slower recovery, more pain, risk of haze (mitigated by MMC 0.02% × 30 seconds).
  2. SMILE (Small Incision Lenticule Extraction): Flapless, femtosecond laser creates intrastromal lenticule removed through small incision. Better biomechanical preservation, less dry eye.
  3. Phakic IOL (ICL — Implantable Collamer Lens): For high myopia (> -10 D) or thin corneas. Reversible. Visian ICL (STAAR).
  4. Refractive Lens Exchange (RLE): Clear lens extraction with premium IOL. For presbyopic patients or extreme refractive errors.
  5. Conductive keratoplasty (CK): Radiofrequency for low hyperopia/presbyopia (rarely performed).
  6. Intracorneal ring segments (ICRS — Intacs, Ferrara rings): For mild keratoconus or post-LASIK ectasia.
10. Complications

Intraoperative:

  • Flap complications: buttonhole, free cap, incomplete flap, irregular flap, thin flap. Incidence reduced significantly with femtosecond laser (< 0.5%).
  • Suction loss during flap creation
  • Decentred ablation

Early postoperative (days to weeks):

  • Diffuse lamellar keratitis (DLK — "Sands of Sahara"): Sterile interface inflammation. Graded I–IV (Linebarger classification). Grade I–II: increase topical steroids. Grade III–IV: lift flap and irrigate interface.
  • Flap displacement/dislocation: Usually from trauma. Requires urgent repositioning.
  • Flap striae: Microstriae (visually insignificant) or macrostriae (reduce BCVA). Lift and reposition/iron/stretch flap.
  • Epithelial ingrowth: Epithelial cells growing under flap edge. Grade 1: observe. Grade 2–3: lift flap and debride.
  • Infectious keratitis: Rare (~1:2,000–1:5,000, approximately 0.02–0.05%). Atypical organisms (Mycobacteria, Nocardia) more common than typical bacteria.
  • Central toxic keratopathy (CTK): Rare, central corneal flattening.

Late postoperative (weeks to years):

  • Dry eye: Most common complication. Due to corneal nerve transection during flap creation. Usually resolves by 6–12 months. Worse with microkeratome than femtosecond.
  • Regression: Gradual return of refractive error, more common in higher corrections.
  • Ectasia (post-LASIK keratectasia): Progressive corneal thinning and steepening. Most feared complication. Risk factors: thin RSB, forme fruste keratoconus, high myopia, young age. Management: CXL (corneal cross-linking), ICRS, eventually keratoplasty.
  • Night vision disturbances: Haloes, starbursts, glare. Due to HOAs, small optical zone, or large scotopic pupil. Reduced with wavefront-guided ablation.
  • Contrast sensitivity reduction.
  • Traumatic flap dislocation: Can occur years after surgery.
11. Management

Pre-operative:

  • Comprehensive screening (topography, pachymetry, wavefront, tear film)
  • Contact lens holiday (soft 1–2 weeks, RGP 3–4 weeks minimum; longer for long-term wearers)
  • Informed consent: discuss realistic expectations, dry eye risk, enhancement possibility
  • Treat pre-existing dry eye before proceeding

Surgical steps:

  1. Topical anaesthesia (proparacaine 0.5%)
  2. Marking of cornea (gentian violet) to assist flap realignment
  3. Flap creation: Femtosecond laser (preferred) — 90–120 μm thickness, 8.5–9.0 mm diameter, superior hinge
  4. Flap lift with spatula
  5. Excimer laser ablation of stromal bed (patient fixates on target light; eye tracker engaged)
  6. Irrigation of interface
  7. Flap repositioning and alignment — ensure no debris, striae, or epithelial defects
  8. Wait 2–3 minutes for flap adhesion

Post-operative regimen (AAO Refractive PPP):

  • Topical antibiotic (moxifloxacin 0.5% QID × 1 week)
  • Topical corticosteroid (prednisolone acetate 1% or fluorometholone 0.1% QID × 1–2 weeks, taper)
  • Preservative-free artificial tears frequently (QID minimum × 3–6 months)
  • Protective shield at night × 1 week
  • Avoid eye rubbing, swimming, contact sports × 2–4 weeks

Follow-up: Day 1, Week 1, Month 1, Month 3, Month 6, Month 12.

Enhancement:

  • If residual refractive error > 0.50–0.75 D after stabilisation (3–6 months)
  • Re-lift flap or perform surface ablation over flap
  • Ensure adequate RSB before enhancement
12. Prognosis

Excellent outcomes for appropriate candidates:

  • Efficacy: ~90% of myopic LASIK patients achieve UCVA 20/20 or better at 12 months (Sandoval et al., 2016: 90.8% across 67,893 eyes); >99% achieve 20/40 or better.
  • Safety: < 1% lose 2 or more lines of BCVA.
  • Predictability: 95–99% within ±1.00 D of target; 85–95% within ±0.50 D.
  • Patient satisfaction: > 95% (PROWL studies, FDA).

Factors affecting outcome:

  • Lower pre-operative myopia has better outcomes (< -6 D optimal)
  • Hyperopic LASIK less predictable and more prone to regression than myopic
  • Wavefront-guided / topography-guided profiles produce better quality of vision
  • Femtosecond laser flap creation reduces flap-related complications

Long-term considerations:

  • Stable refractive results for most patients over 10+ years
  • Presbyopia onset (age 40–45) will require reading glasses regardless
  • Post-LASIK IOL power calculation is complex (altered keratometry). Use Haigis-L, Barrett True-K, or ASCRS post-refractive IOL calculator.
  • Ectasia risk: very low (estimated 1 in 2,500–5,000+ with modern screening, 0.03–0.09%; older data cited 1 in 2,500) but can occur years later
  • Enhancement rate: 5–15% depending on initial correction

Clinical Pearls

1
The minimum residual stromal bed (RSB) must be ≥ 250 μm — this is the most critical safety threshold in LASIK to prevent post-operative ectasia.
2
Always obtain a Pentacam / Scheimpflug scan — posterior corneal elevation and the Belin-Ambrósio Display (BAD-D) are the most sensitive screening tools for subclinical keratoconus before refractive surgery.
3
DLK ('Sands of Sahara') is a sterile interface keratitis that must be distinguished from infectious keratitis — DLK is diffuse and non-focal, while infection is focal with overlying epithelial defect.
4
Post-LASIK patients have falsely low IOP readings on Goldmann applanation due to thinner corneas and altered biomechanics — always use corrected IOP or dynamic contour tonometry in these patients.
5
IOL power calculation after LASIK is notoriously inaccurate using standard formulas — always use the ASCRS post-refractive surgery calculator or Barrett True-K formula, and collect pre-LASIK keratometry if available.
6
A contact lens holiday (soft 1–2 weeks, RGP 3–4 weeks) before pre-operative evaluation is essential — CLs alter corneal curvature and can lead to erroneous topography and refraction.
7
Femtosecond laser flap creation has largely replaced microkeratome — it produces a planar flap of uniform thickness with fewer flap complications (< 0.5%).
8
Exam trap — LASIK absolute contraindications: Keratoconus (or forme fruste KC on topography), unstable refraction (changing >0.5D/year), corneal thickness insufficient for ablation (residual stromal bed <250μm), active autoimmune disease (RA, SLE — risk of corneal melt), uncontrolled dry eye, pregnancy/breastfeeding. Topography is MANDATORY — missing subclinical KC leads to post-LASIK ectasia.
9
Exam trap — Post-LASIK ectasia: Progressive corneal thinning and steepening after LASIK, resembling keratoconus. Risk factors: thin cornea, high myopia correction, residual bed <250μm, abnormal topography missed preoperatively. Treatment: corneal collagen cross-linking (CXL) halts progression. Ectasia is the most feared long-term complication of LASIK.
10
Exam trap — DLK (Diffuse Lamellar Keratitis): 'Sands of Sahara' — diffuse inflammatory infiltrate at the LASIK flap interface. Graded 1–4 (Stage 4: central involvement with stromal melt). Stage 1–2: intensive topical steroids. Stage 3–4: flap lift and irrigation + intensive steroids. Usually occurs days 1–5 post-LASIK. Differentiate from infectious keratitis (DLK is sterile, diffuse; infection is focal, progressive).

Oral-exam questions

  • What is the minimum residual stromal bed thickness in LASIK? — 250 μm. Below this threshold, the risk of post-LASIK ectasia increases significantly (AAO Refractive PPP).
  • What is the Munnerlyn formula? — Ablation depth = S² × D / 3, where S = optical zone diameter (mm) and D = dioptres. Approximately 12–16 μm of tissue per dioptre of myopic correction.
  • What is DLK and how do you grade it? — Diffuse lamellar keratitis ('Sands of Sahara') is sterile interface inflammation graded I–IV (Linebarger). Grades I–II: increase steroids. Grades III–IV: lift flap and irrigate.
  • How does LASIK cause dry eye? — Flap creation transects the sub-basal corneal nerve plexus, reducing corneal sensation and reflex tearing. Typically resolves by 6–12 months. SMILE causes less dry eye because it preserves more nerves.
  • What is the PROWL study's key finding? — The PROWL studies (FDA) showed 95% patient satisfaction after LASIK. Up to 46% develop at least one new visual symptom (mostly mild) at 3 months (Eydelman et al., JAMA Ophthalmol 2017).
  • How do you calculate IOL power after LASIK? — Standard formulas overestimate corneal power (altered K-readings). Use ASCRS post-refractive calculator, Barrett True-K, or Haigis-L. Pre-LASIK keratometry is invaluable if available.
  • What is the most feared long-term complication of LASIK? — Post-LASIK ectasia. Progressive corneal thinning and steepening, risk factors include thin RSB, subclinical keratoconus, high myopia. Manage with CXL (corneal cross-linking).
  • Why is residual stromal bed thickness <250μm dangerous after LASIK? — The LASIK flap (typically 90–110μm) does NOT contribute significantly to corneal biomechanical strength — the residual stromal bed bears the load. If the bed is =<250μm, the remaining cornea is too thin to resist normal IOP, leading to progressive forward bulging (ectasia), resembling post-surgical keratoconus. The 250μm threshold is the consensus minimum, but many surgeons prefer ≥300μm== for safety. Note: RSB is a continuous risk variable — ectasia has occurred with RSB > 250 μm (~50% of reported ectasia cases), and other factors (topography, biomechanics, age) contribute to overall risk.

Mnemonics

FLAP (LASIK Contraindications)

F — Forme fruste keratoconus / thin cornea L — Less than 250 μm residual stromal bed A — Autoimmune disease (uncontrolled) P — Pregnant or unstable refraction

DLK Grades (1-2-3-4)

1 — Peripheral granules → increase steroids 2 — Extends centrally → hourly steroids 3 — Central clumping, VA down → lift and irrigate 4 — Stromal melt → urgent intervention

Comparison Tables

LASIK vs PRK vs SMILE — Key Differences
Flap/access
LASIK (Femto-LASIK)
Hinged flap (90–120 μm)
PRK
No flap — surface ablation
SMILE
No flap — small incision (2–4 mm)
Laser used
LASIK (Femto-LASIK)
Femtosecond + excimer
PRK
Excimer only
SMILE
Femtosecond only
Visual recovery
LASIK (Femto-LASIK)
Fastest (hours to 1 day)
PRK
Slowest (3–5 days pain, weeks for VA)
SMILE
Intermediate (1–7 days)
Dry eye
LASIK (Femto-LASIK)
Moderate (nerve transection)
PRK
Moderate (nerve ablation)
SMILE
Least (nerve preservation)
Biomechanics
LASIK (Femto-LASIK)
Most weakened (flap cuts anterior stroma)
PRK
Moderate (anterior stroma ablated)
SMILE
Best preserved (anterior stroma intact)
Ectasia risk
LASIK (Femto-LASIK)
Highest (if RSB inadequate)
PRK
Lowest
SMILE
Intermediate
Hyperopia correction
LASIK (Femto-LASIK)
Yes
PRK
Yes
SMILE
No (currently)
Wavefront-guided
LASIK (Femto-LASIK)
Yes
PRK
Yes
SMILE
No (currently)
Enhancement
LASIK (Femto-LASIK)
Easy (re-lift flap)
PRK
Surface ablation again
SMILE
Complex (Circle procedure or PRK over cap)
DLK Grading (Linebarger Classification) and Management
I
Appearance
White granular cells in periphery
Location
Interface periphery only
Management
Increase topical steroid frequency
II
Appearance
White granular cells extending centrally
Location
Interface periphery + centre
Management
Increase steroid to hourly; close follow-up
III
Appearance
Dense central clumping, reduced BCVA
Location
Central interface, aggregated
Management
Lift flap + irrigate interface + intensive steroids
IV
Appearance
Stromal melting, permanent damage
Location
Central with corneal thinning
Management
Urgent flap lift + irrigation; may need further intervention
Ablation Profiles in LASIK
Conventional
Basis
Sphero-cylindrical Rx
Advantage
Simple, reliable
Best For
Low-moderate myopia without significant HOAs
Wavefront-guided
Basis
Aberrometry (HOA map)
Advantage
Corrects HOAs; fewer night symptoms
Best For
Patients with significant HOAs; night vision concerns
Wavefront-optimised
Basis
Pre-compensates for induced SA
Advantage
Less spherical aberration induction
Best For
Routine myopia without pre-existing HOAs
Topography-guided (Contoura)
Basis
Corneal topography data
Advantage
Corrects corneal irregularity
Best For
Irregular corneas; enhancements; forme fruste KC

Self-Assessment (5)

MCQ

A 28-year-old woman with myopia of -4.00 D has a central corneal thickness of 470 μm. Pentacam shows suspicious posterior elevation. What is the most appropriate next step?

MCQ

A patient develops diffuse white granular deposits at the flap interface 2 days after LASIK, with best-corrected visual acuity of 20/20 and deposits confined to the periphery. What is the diagnosis and management?

MCQ

What is the approximate tissue ablation depth per dioptre of myopic correction in LASIK with a 6.0 mm optical zone?

MCQ

According to the PROWL studies (FDA), what percentage of LASIK patients reported being satisfied with their outcomes?

MCQ

A 35-year-old LASIK patient presents 3 years post-operatively with progressive myopic shift and increasing irregular astigmatism. Topography shows inferior steepening. What is the most likely diagnosis?

References

  1. AAO Preferred Practice Pattern: Refractive Errors & Refractive Surgery (2018)
  2. Eydelman MB, et al. Symptoms and satisfaction of patients in the Patient-Reported Outcomes With LASIK (PROWL) studies. JAMA Ophthalmol. 2017;135(1):13-22.
  3. Sandoval HP, et al. Modern laser in situ keratomileusis outcomes. J Cataract Refract Surg. 2016;42(8):1224-1234.
  4. Sekundo W, et al. Small incision corneal refractive lenticule extraction (SMILE) for the correction of myopia. J Cataract Refract Surg. 2011;37(1):127-137.
  5. Randleman JB, et al. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37-50.
  6. Linebarger EJ, et al. Diffuse lamellar keratitis: diagnosis and management. J Cataract Refract Surg. 2000;26(7):1072-1077.
  7. Steinert RF. Cataract Surgery, 3rd Edition. Saunders/Elsevier.
  8. Kanski JJ, Bowling B. Clinical Ophthalmology: A Systematic Approach, 9th Edition

Related Content

PROWL — Patient-Reported Outcomes With LASIKSMILE vs LASIK (Sekundo et al.)PERK Study — Prospective Evaluation of Radial KeratotomySMILEKeratoconusDry Eye DiseaseAge-Related Cataract (post-LASIK IOL calculation)
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